Evaluation of Spring Wheat Genotypes for Climatic Adaptability using Canopy Temperature as Physiological Indicator

C change is cumulative accepted as a worldwide phenomenon with possibly significant consequences (IPCC, 2007b) and accompanying with recurrent extreme weather events (Stern, 2006; Karl et al., 2009). Global average annual temperature has increased by ~0.4 C since 1980, with some extreme changes in different parts of the world (IPCC, 2001) Many studies have indicated the impacts of upcoming climate changes on food security (Rosenzweig and Parry, 1994; Edmonds and Rosenberg, 2005). Third world countries are only contributing 10% to the annual global carbon dioxide emissions but are most Abstract | Temperature and drought stresses are generally considered main wheat yield limiting factors in rainfed environment, especially during reproductive growth stages. However, variability might exist among wheat genotypes in response to stressful conditions. The objective of the study was to test the climatic adaptability potential of the wheat genotypes using canopy temperature measurement as physiological indicator. The trial was comprised of forty spring wheat genotypes which were sown on normal and late planting dates to create variable growing environments under field conditions. Wheat genotypes showed significant variation (p<0.05) under both normal and late planting dates. The genotypes showed a strong correlation among canopy temperature, spike sterility and grain yield under both planting dates. The canopy temperature and grain yield correlated negatively under both normal (r2 = -0.943) and late planting (r2 = -0.957). However, the canopy temperature and spike sterility showed strong positive correlations under both normal (r2 = 0.920) and late (r2 = 0.937) plating dates, respectively. The results indicate that genotypes viz; Pakistan-2013, DN-93, SRN-09111, PR-103, NR-409, NR-421 (now approved wheat variety in the name of Zincol-2016), Galaxy and NARC-2011 maintained low crop canopy temperature during grain filling period showed lower spike sterility and produced higher grain yields. The genotypes with higher canopy temperature during grain filling period showed higher spike sterility which is linked to lower grain yield. The wheat genotypes identified with better climatic adaptability are potentially useful sources for improving drought and heat tolerance in wheat. Moreover, canopy temperature measurement proved to be a useful phenotyping tool in selection of drought and heat tolerant wheat germplasm under field conditions. Muhammad Sohail1*, Imtiaz Hussain2, Maqsood Qamar2, Sikander Khan Tanveer2, Syed Haider Abbas2, Zeshan Ali1 and Muhammad Imtiaz3

susceptible to climate change (Maskrey et al., 2007) because they are reliant on mainly agriculture-based rural livelihood (World Bank, 2009). Crops cultivation in Pakistan are mainly dominated in arid and semiarid regions ( Janjua et al., 2010) and changes in rainfall pattern and increase in temperature would adversely impact crop productivity.
Shift in seasonal temperature and rainfall patterns forecast the global climate variability. There is high level of uncertainty about the nature of micro climate change. High temperature and limited soil moisture adversely affect physio-agronomic traits of the crop under rainfed conditions, which ultimately led to lower grain yield (Sohail et al., 2013). Reduced grain yield under rainfed conditions might be linked to grain mortality (spike sterility) due to high temperature injury at reproductive growth stages of the crop (Calderini et al., 1999;Khan et al., 2007). Terminal heat stress shortens grain filling period, disrupt synthesis of starch in the endosperms of the kernels, induce early maturity which cause shrinking of kernels, reduce grain weight (Rawson, 1987;Nageswara et al., 2001) and quality (Sohail et al., 2018).
Wheat occupies the largest rainfed area among all the crops (Portmann et al., 2010). Significant variations exist among wheat genotypes in terms of their adaptability to stressful environment (Khan et al., 2007;Mehboob et al., 2005;Okuyama et al., 2005). The ability of the wheat genotypes to withstand high temperature stress is associated with various physiological adaptability traits (Lopes and Reynolds, 2010;Renolds et al., 1994). Adapted genotypes can play important role to enhance wheat productivity under rainfed conditions. It is a daunting task for the wheat breeders to increase the productivity under stress environment. Selection for tolerance to temperature and drought under field conditions is complicated due to unavailability of quick screening techniques to evaluate large number of genotypes. Many physiological and agronomic indices to assess yield loss under drought and heat tolerance have been used for evaluation of tolerant genotype. However, Infrared thermometry (canopy temperature measurements) have been widely used in recent years to study genetic variation among genotypes under field conditions in response to temperature and drought. Crop canopy temperature within and near the surface of canopy is more or less different from the air temperature. Canopy temperature can be warmer and cooler than the surrounding air depending upon the prevailing micro environmental factors (Sohail et al., 2014). During transpiration process, a significant amount of energy is needed to transform liquid water into vapour, and this energy is consumed by evaporating water from the leaf which lowers the leaf temperature and surrounding air which links canopy temperature with crop water stress and evapotranspiration. Canopy temperature measurement can be a good indicator of abiotic stress and metabolic functioning of plant (Colaizzi et al., 2012).
Infrared thermometer (measuring crop canopy temperature) is used as remote sensing tool which is also named as thermometry, thermal sensing or thermography. These spectral reflectance techniques are found to be very convenient and robust to estimate abiotic stresses in plants (Renold, 1998;Mahan and Yeater, 2008;Vadivambal and Jayas, 2011). A handheld infrared thermometer is a non-destructive, quick and robust tool to measure canopy temperature (CT) which is related to the amount of transpiration resulting in evaporative surface cooling (Renold, 1998). Researchers have reported a strong correlation among canopy temperature measurement with many physiological factors: stomatal conductance, transpiration rate, plant water status, water use, leaf area index and crop yield (Khan et al., 2007;Sohail et al., 2013). Variation in canopy temperature of plant species in contrast to prevailing air temperature has been documented as indicators of overall plant water status (Blum et al., 1982;Jackson et al., 1981) and used in evaluation of plant species for drought stress tolerance (Blum et al., 1989;Royo et al., 2002;Rashid et al., 1999). Canopy temperature measurement is also used as physiological tool for screening large populations of wheat against heat stress. (Amani et al., 1996;Reynolds et al., 1998). Fischer et al. (1998) observed strong association of cool crop canopy temperature maintenance under stressful environment with grain yield increase in wheat genotypes.
The study was conducted to evaluate temperature and drought stress adaptability potential of wheat genotypes in rainfed environment using canopy temperature measurement as physiological indicator. The research work is a step towards generation of useful information for crop improvement under abiotic stresses. (late). The late sowing was used to expose the crop to more extreme weather conditions. Experimental site comprised clay type soil with low organic matter (0.8%) and low amounts of N and P. The pH was 7.7 without any salinity problems.

Materials and Methods
Land preparation was done by using disc plow (primary tillage) and planking (secondary tillage). The crop was planted with self-propelled wheat planter at seed rate of 120 kg ha -1 . Sowing for each genotype was done in 6-row plots, 5 m long, with a 25 cm row spacing. Nitrogen and phosphate fertilizers were applied in ratio of 120: 85 respectively, in form of Urea and DAP. Full dose of nitrogenous and phosphate fertilizers were applied at planting, which is regular practice under rainfed conditions. Daily mean, minimum and maximum temperatures at the experimental site were recorded along with relative humidity (RH) ( Table 1).
A hand-held Infrared Thermometer (Model AG-42, Telatemp Crop, Fullerton, CA.) was used to measure canopy temperature. Canopy temperature (°C) reading were recorded at boot (Feekes 10), Kernel watery ripe (Feekes 10.5.4) and mealy ripe (Feekes 11.2) growth stages of the crop. Measurements were taken by holding the Infrared Thermometer at an appropriate angle (30° from the horizontal and approximately 50 cm above the canopy) and 1 m distance from the edge of the plot to avoid the effect of soil temperature. One measurement per plot was taken between 11:00 and 14:00 hours under calm air conditions (Reynold et al., 1998). Spike sterility (%) in the field was observed visually as gaping glume, transparent florets and calculated as average percentage of randomly selected 15 spikes per plot (not clear). Grain yield was measured after harvesting, threshing and weighing of grains of 2 m 2 sample area which was converted into kg ha -1 . The data were tested for analysis of variance using Statistix v. 7.0 package. Treatment means were compared using Tukey's HSD test at P ≤ 0.05. Correlations among grain yield, spike sterility and canopy temperature (°C) were determined using MS Excel 2007.

Results and Discussion
Interactive study of sowing dates and genotypes showed significant variation (p<0.05) in terms of canopy temperature, spike sterility and grain yield. A negative correlation was observed between crop canopy temperature and grain yield under both normal (r 2 = -0.943) and late (r 2 = -0.957) growing conditions. At the same time, significant variation was noticed within the entries tested. Under normal growing conditions, the group of entries that maintained mean canopy temperature (CT) less than 23 °C during grain filling period produced grain yield more than 4500 kg ha -1 , on other hand, the entries that showed mean canopy temperature higher than 25 °C produced grain yield less than 4000 kg ha -1 (Figure 1). In case of late planting, which exposed crop to more adverse growing conditions at grain filling period as compared to normal sowing, the group of entries which maintained canopy temperature below 24.5 °C during grain filling period produced more than 3000 kg ha -1 while entries with mean canopy temperature higher than 24.5 °C produced grain yield less than 3000 Kg ha -1 (Figure 2).  A positive correlation was also observed between canopy temperature and spike sterility under both normal (r 2 = 0.920) and late sowing (r 2 = 0.937). Under normal growing conditions, the entries which maintained mean canopy temperature less than 24 °C during grain filling period showed sterility percentage less than 10 %. The genotypes that showed mean canopy temperature higher than 24 °C showed more than 10% spike sterility (Figure 3). Spike sterility percentage increased in the late plantation. However, variation was observed among the tested genotypes. The group of genotypes which maintained mean canopy temperature less than 25 °C showed spike sterility percentage less than 15%. While, entries with canopy temperature higher than 25 °C showed more than 15% spike sterility (Figure 4).  All genotypes showed lower grain yield and higher spike sterility percentage under late plantation. The higher spike sterility among late sowing genotypes was due to the exposure to more adverse weather conditions.
Results of this study clearly showed that the entries which managed to maintain low mean canopy temperature contributed to higher grain yield. Postanthesis high temperature stress and drought seems responsible for higher floret sterility and grain mortality (Sikder and Paul, 2010). Abortion of kernels at high temperature may be due to reduced photosynthetic activity and decreased supply of carbohydrates which leads to increased spike sterility. Reduced growth period and increased spike sterility significantly lowered grain yield of wheat genotypes under late growing conditions than normal planting (Reference?).
However, significant variation among genotypes in relation to spike sterility and grain yield under both normal and late plantation indicated that some genotypes were more adapted to rainfed conditions. Crop canopy temperature has been shown to be well correlated to spike sterility and grain yields of wheat genotypes. For instance, entries with lower mean canopy temperature showed less spike sterility and higher grain yields as compared to others, which may be their inherent character to tolerate temperature and drought stress especially under late planting conditions. Significant variation among genotypes was observed by many researchers under variable growing conditions ( Jain et al., 1992;Okuyama et al., 2005;Kumar et al., 1994).
Maintenance of low canopy temperature by genotypes under stressful rainfed conditions may be linked to their ability to extract water through better root system (Lopes and Reynolds, 2010) and larger stomatal conductance (Reynolds et al., 2005). Low canopy temperature of random wheat lines was associated with higher grain yields in hot environments (Lopes and Reynolds, 2010;Sohail et al., 2013). Variation in canopy temperature maintenance among wheat genotypes might be due to their genetic differences which are also mentioned by Renolds et al. (1994) that genotypic variations existed for canopy temperature among the wheat germplasms under climatic stress condition.
Cool canopy maintenance of wheat genotypes during grain filling duration is an important physiological indication for terminal heat stress tolerance (Munjal and Rena, 2003). These findings showed that increase in crop canopy temperature at grain filling period in susceptible genotypes reduced biomass accumulation and grain yield. Similar results were also reported by Munjal and Rena (2003) that cool canopy during grain filling time is a vital physiological principle for heat stress tolerance in wheat. Canopy temperature measurement has been used as effective selection criteria for evaluating screening wheat cultivars for heat tolerance and proved most potential screening (Reynolds et al., 2001).
So, wheat breeders can opt crop canopy temperature measurement as efficient selection criteria for heat tolerant wheat genotypes which will provide basis for developing climate resilient wheat varieties.

Conclusions and Recommendations
Wheat crop under late sowing condition was more exposed to high temperature stress during grain filling period which caused grain mortality and lead to reduced grain yield among all genotypes. However, genotypes showed variability under stressful rainfed conditions. Some genotypes i.e. Pakistan-2013, DN-93, SRN-09111, PR-103, NR-409, NR-421 showed higher grain yields under both normal and late growing conditions and proved to be more climate adapted than others. The study generated useful information to be utilized in wheat improvement programs with the aim of developing drought and heat tolerant wheat material. Moreover, this study revealed crop canopy temperature as a pretty useful physiological trait to estimate drought and temperature stress tolerance potential of wheat genotypes.